1. Field of the Invention
The present invention relates to a voice signal processing system and a voice signal processing method in a voice communication system mounted with an echo canceller.
2. Description of the Related Art
In recent years, an adaptive echo canceller whose representative is the ITU-T G. 168 standard is used in a voice communication circuit adopting TDM (time division multiplexing) system such as a PBX (private branch exchange).
Among conventional art related to the present invention is Japanese Patent Laying-Open (Kokai) No. Heisei 6-216811. Japanese Patent Laying-Open (Kokai) No. Heisei 6-216811 discloses a technique of, noticing signal waveform distortion generated because when transmitting a voice signal of an excessive level in a circuit including an echo canceller, the voice signal level exceeds a dynamic range of a circuit inside an echo path, controlling a maximum signal level to fall within the dynamic range to suppress the signal waveform distortion, thereby enabling the echo canceller to exert its performance.
As recited in the above-described Japanese Patent Laying-Open (Kokai) No. Heisei 6-216811, however, simple control of an excessive voice signal level to prevent the level from exceeding a dynamic range makes a voice waveform be a trapezoidal form, resulting in that the echo canceller can not cancel echoes satisfactorily. This is because when a signal waveform applied to an echo path is trapezoidal, signal waveform distortion is caused due to transient response characteristics of a circuit (D/A converter etc.) in the echo path. In other words, also the signal waveform distortion caused by transient response characteristics prevents operation of the echo canceller.
The above-described Japanese Patent Laying-Open (Kokai) No. Heisei 6-216811 thus conducts only signal level limitation without taking transient response characteristics into consideration, so that it is hard to say that suppression of signal waveform distortion can be satisfactorily realized. In other words, because nonlinear signal waveform distortion caused due to transient characteristics of CODEC (coder/decoder) in the echo path hinders operation of an adaptive echo canceller, echoes can not be satisfactorily cancelled.